For example, DE 42 40 491 A1 appears to disclose a filling level measuring device of the initially mentioned kind that work on the transit time principle and emit for example microwave signals. For processing the reflected and received microwave signals, various analog circuits or components are nowadays available so as to generate a so-called envelope curve, which may then be digitized for assessing the filling level height here from. The generation of an envelope is sufficiently known, and accordingly, there is no need for further explaining this technology (e.g. cf.: DE 44 07 396 C2; M. Scholnick, “Introduction to Radar Systems”, 2nd ed. 1980, McGraw-Hill; Peter Devine, “Radar Level Measurement—The User's Guide” VEGA Controlls Ltd., 2000, ISBN 0-9538920-0-X).
In the present technical field, one works with microwave signals having a frequency of for example 1 up to 30 GHz or more. Due to these high frequencies, various analog circuits or components have been necessary to date for determining the exact filling level from the received microwave signals. In particular, so-called “mixers” are inter alia used, by means of which a temporal “lengthening” of the envelope is performed, so that the sampling of the envelope curve with usual A/D converters (pulse frequencies of, for example, about 25 microseconds) with a sufficiently fine sampling lattice (for achieving a sufficient accuracy of the determined filling level height) becomes possible. This means that in particular microwave pulses are transformed by means of a mixer and an associated oscillator into another frequency and time domain. Hereby, pulses are, for example, lengthened in time after the time transformation by about the factor 160,000, whereby a pulse duration of 1-2 nanoseconds prior to the transformation corresponds to a pulse duration of about 160-320 microseconds after the transformation. These time-lengthened pulses are then supplied to the A/D converter, which samples this signal in a sampling lattice of about 25 microseconds. Thus, with a pulse length of 160 microseconds, about six sampling values are available per pulse. With these values, accuracies of about 5 mm may be achieved in the filling level height determination. It has to be pointed out that the accuracy of the filling level height determination depends also on the magnitude of the sampling values per echo pulse, on the steepness of the pulse slope and on the accuracy of the A/D converter.
In summary, it has to be noted that devices for determining the filling level height on the basis of reflected microwaves, in particular microwave pulses, perform an analog processing of the received signals so as to be able to achieve the desired accuracy in the determination of the filling level height. The use of analog components or analog circuit technology, however, can be subject to disadvantages. Thus, in the dimensional configuration of the circuit, component tolerances and manufacturing divergences of the components have to be taken into account, and also have to be monitored during the production. Analog components moreover exhibit temperature dependencies, which have to be considered and cause a rather significant test effort. Temperature dependencies and component tolerances, in addition, can result in a decrease of the reproducibility and accuracy of the output signals, and have to be minimized or compensated by an additional effort. An disadvantage may also to be seen in that modifications in the functional mode of such analog circuits can only be performed at an important effort. Finally, analog circuits may be also sensitive for interfering influences.